Male octopuses usually carry their 3rd right arm curled, the tip is modified (not always obvious and does vary within species from descriptions I've seen), over-sized suckers on certain arms is the easiest way to tell in my opinion. My O. briareus for example has obviously larger suckers closer to the mouth, about 3 or 4 inches down there is a dramatic decrease in sucker size. Females seem to have a more uniform appearance to their suckers.